This invention pertains to structure including power boost mechanism for converting linear throttle input commands from a pilot to rotary motion and amplifying output torque for control of an engine fuel control. This structure takes the form of a rack assembly having the capability for three different modes of operation. In a boost drive mode, a power boost mechanism provides boost power to the pilot's linear input command thereby lessening the work load of the pilot when operating the throttle control. In a direct drive mode, the pilot can obtain direct operation when there is a loss of boost power by a direct mechanical drive to the engine fuel control. Additionally, there is a shift mode whereby the controlling output shaft can be automatically advanced or retarded in the event there is a fuel control system failure.
Structure for providing a boost drive mode is known, with the prior art including U.S. Pat. Nos. 2,750,928 and 3,011,482. Use of a jet-pipe valve for controlling actuation of a cylinder is shown in U.S. Pat. No. 3,433,133. Also known is structure whereby there can be shift to direct manual operation on loss of fluid pressure for a power boost system, as shown in U.S. Reissue Pat. No. 23,490.